Open core potential transformer



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Apr1] 3, 1962 G. CAMlLLl ETAL OPEN CORE POTENTIAL TRANSFORMER 5 SheetsSheet 3 Filed Aug. 51, 1959 United States Patent 3,028,569 OPEN CORE POTENTIAL TRANSFORMER Guglielmo Camilli, Pittsfield, and Edward R. Uhlig, Becket, Mass., assignors to General Electric Company, a corporation of New York 7 Filed Aug. 31, 1959, Ser. No. 837,019 16 Claims. (Cl. 336-70) This invention relates to stationary electrical induction apparatus, and more in particular to an improved high voltage potential transformer for use in high voltage transmission line systems for the operation of relays and for the measurement of electrical power.

In high voltage electrical systems it is frequently desired to provide protective and indicating systems responsive to the voltage in the system. Since the extremely high voltages encountered in such systems are not utilizable directly for such purposes, it has been common to employ potential transformers connected to the systems and providing a low voltage output proportional to the voltage of the system. One type of previously employed potential transformer was in essence a miniature power transformer, employing a single high voltage winding and a single low voltage winding. Such transformers required full insulation between the low voltage and the high voltage wvinding, and consequently units built in this manner were bulky and costly. As an alternative, a more economical arrangement was found to exist in the use of a number of separate smaller transformers inter-connected in such a manner that the total system voltage did not appear across any one of the transformers. Such an arrangement is commonly known as a cascade transformer, and results in a substantial decrease in the cost since the high and low voltage windings of each separate unit in the transformer need to be insulated only for a fraction of the total system voltage. Although such arrangements lend themselves to a transformer which is less bulky and less costly than a single stage unit, such as has been previously employed, the necessity for separately mounting a number of core and coil assemblies was diflicult and costly.

It is therefore an object of this invention to provide an improved potential transformer.

Another object of this invention is to provide an economical potential transformer for use in conjunction with high voltage transmission line systems, the transformer being characterized by the fact that full insulation is not required as in the case of a single stage potential transformer, and only a single transformer core unit is required as contrasted to the cascade transformer type.

Briefly stated, the invention comprises a potential transformer having an open magnetic core comprised of a stack of flat magnetic strips. A low voltage winding is provided surrounding the stack, and the low voltage winding may be divided into a number of axially separated sections. A high voltage winding is provided surrounding the low voltage winding, the high voltage winding comprising also a plurality of axially spaced sections, which may be radially aligned with the low voltage sections. The high voltage winding sections are serially connected, and a high voltage terminal is connected to one of the axially end-most high voltage sections. An electrostatic shield is provided surrounding each of the remaining sections of the high voltage windings, the shields being connected to the next higher voltage sections. The insulation and spacing between the high and low voltage winding groups is increased progressively as the voltage increases and reaches a maximum in the group to which the high voltage terminal is connected.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which we regard as our invention, it is believed that ice the invention will be better understood from the following description taken in connection with the accompanying drawings.

In the drawings:

FIG. 1 is a broken away view of a potential transformer and illustrating one embodiment of the invention.

FIG. 2 is a cross-sectional view of the core and low voltage winding cylinder of the transformer of FIG. 1, the section being taken transversely of the axis of the core.

FIG. 3 is an enlarged cross-sectional view of a portion of the transformer of the FIG. 1, and'illustrating the winding, core and insulation arrangement of one embodiment of this invention.

FIG. 4 is a circuit diagram illustrating the effective capacitance network of the high voltage winding and capacitance pads of the potential transformer of FIGS. 1 and 3.

FIG. 5 is an enlarged cross-sectional view of a modified form of the potential transformer of FIGS. 1 and 3, and

FIG. 6 is a graph illustrating the comparative effects of the voltage distribution in the high voltage winding of a transformer of the type illustrated in FIGS. 1 and 3.

Referring now to the drawings and more in particular in FIG. 1, therein is illustrated a potential transformer comprising a plurality of stacked hollow insulating shells 10 mounted on a base 11, and covered by an upper conducting cap 12. The shells 10, base 11, and cap 12 define a sealed chamber 13 having a general vertical elongation, and which may be filled with a dielectric fluid such as transformer oil.

A magnetic core 14 is provided generally centrally disposed within the insulating shells 10, and preferably having a vertical axis. The core 14 may be comprised of a stack of flat strips of magnetic material. As illustrated in FIG. 2, the magnetic core 14 may have a generally cruciform cross section, as is the conventional practice in magnetic cores, in order to permit the use of a greater amount of magnetic material within a circular coil. The core 14 is radially surrounded by an insulating cylinder 15 over which is wound a low voltage winding 16. The low voltage winding 16 may be separated into a plurality of axially separated groups, as will be disclosed in more detail in the following paragraphs. In order to reduce the dielectric stresses at the ends of the low voltage winding and the exposed edges of the magnetic core, a hemispherical conductive cap 20 (which may be more clearly seen in FIG. 3) may be provided at the upper (high potential) end of the core, the shield 20 being electrically connected to the core and extending radially at least as far as the radially outer extremities of the low voltage winding.

Referring to FIGS. 1 and 3, an insulating cylinder 25 is provided surrounding the low voltage winding. The cylinder 25 is closed at its upper end, and preferably shaped to fit tightly over the hemispherical conducting shield 20. The insulating cylinder 25 may be made of paper pulp material, fabricated according to the method described in U.S. Letters Patent 2,290,671, which issued on an application of G. Camilli and is assigned to the assignee of the present application.

A high voltage winding is provided surrounding the insulating cylinder 25, the high voltage winding being comprised of a plurality of axially spaced apart groups 34 Each of the groups 30 is preferably of the barrel wound type, i.e., comprising a plurality of concentric layers of an electrical conductor, with each layer having a plurality of axially aligned turns. In the example of the invention illustrated in FIG. 3, the high voltage winding is illustrated as being divided into four of the groups 30.

It will be understood, of course, that more or less of such groups may be employed without departing from the spirit or scope of the invention. For the sake of convenience in disclosing the invention, the groups 30 have been designated as groups 31, 32, 33, and 34, progressively from the upper high voltage end of the winding. The winding groups 31 to 34- are serially connected, and the end of the lower or low voltage winding group 34 is connected to ground.

Referring still to FIG. 3, the lower voltage winding 34 is insulated from the insulating cylinder 25 by means of a flanged insulating element or cylinder 36 of predetermined length and thickness. The insulating cylinder 36 extends co-extensively with all of the groups of the high voltage winding, and is provided with a radially outwardly extending flange 37 at its upper end, near the end of the low voltage winding. The winding group 33 is further insulated from the low voltage winding by an insulating cylinder 38, the cylinder 38 surrounding the cylinder 36 and having a lower radially outwardly extending flange betweenthe windinggroups 33 and 34 and an upper radially outwardly extending flange above the upper winding group 31. The cylinder 33 thus also separates the winding groups 32 and 31 from the low voltage winding. Thewinding group 32 is still further insulated from the low voltage winding by means of an insulating barrier 39 surrounding thecylinder 38. The cylinder 38 has a lower radially outwardly extending flange separating the winding groups 32 and 33, and an upper outwardly radially extending flange above the winding group 31, so that the insulating cylinder 39 also separates the high voltage winding group 31 from the low voltage winding. The high voltage winding group 31 is still further insulated from the low voltage winding by means of an insulated cylinder 40 surrounding the insulating cylinder 39. The insulating cylinder 40 has a lower radially extending flange separating the winding groups 31 and 32, and upper radially outwardly extending flange above the winding group 31.

The insulating cylinders separating the high voltage winding groups from the low voltage winding may conveniently be made from compressed crepe paper sheets, and the flanges may be formed, for example, by the arrangement disclosed in U.S. Letters Patent 2,359,544, which issued on an application of G. Camilli and is assigned to the assignee of the present application. The low voltage winding may be split into axially separated groups aligned with respective high voltage groups in order to improve the electric and magnetic characteristics of the transformer.

To state the relationship between the insulating elements or cylinders in other words, each of the insulating elements 36, 38, 39, and 40 has an axial bore therethrough, and the insulating elements are progressively nested one within the bore of another to provide a radially.

expanding insulating means that progressively increases the spacing and insulation between the high and low voltage Winding groups as the voltage within the transformer increases. The nested insulating elements may be made progressively shorter in length, so that the element 40, which has the largest bore, is the shortest in length. The nested insulating elements, because of their decreasing lengths, define radial ledge portions on the exterior of each insulating element, so that the high voltage winding groups may conveniently surround the insulating elements adjacent their radial ledge portions.

One end of the axially endmost winding group 31 is connected by means of a lead 45 to a high voltage terminal 44- disposed in the cap 29. As has been previously stated, each group of the high voltage winding consists of several layers of conducting turns insulated from each other. Each of the winding groups is terminated with an inner annular electrostatic shield 46, substantially axially co-extensive with the winding group, and an outer electrostatic shield 47, also axially co-extensive with the winding group. The shields 46 and 47 are connected respectively, to the inner and outer ends of the winding groups.

As has been stated previously, the outer surfaces of the transformer may be comprised of a plurality of axially disposed insulating shells 10. The shells It? may be held together by any conventional means, such as annular metallic flanges 5'9 cemented to the respective ends of the shells, and bolted together. To relieve high electrostatic stresses caused by such clamping arrangement, annular grading rings 51 may be provided externally of the shells and connected to the flanges 50. The flanges, and hence the grading rings, are connected to the Winding groups 31-34 by means of leads 52 extending between adjacent shells.

Referring still to PEG. 3, each of the three lower high voltage windings is surrounded by an annular electrostatic shield or pad 55. The pads may, for example, be comprised of annular conducting members surrounded by insulation and spaced radially from the electrostatic shields 47. The highest voltage winding 31 is not provided with a pad. The conducting part of each pad is electrically connected to the highest voltage end of the next highest voltage winding group. Thus, for example, the pad 55 surrounding the winding group 32 is connected to the radially outer end of the winding group 31, and the radially inner end of the Winding group 31 is connected to the radially outer end of the winding group 32, so that the full voltage across the winding group 31 appears between the shield &7 of winding group 32 and the pad 55 of winding group 32.

Referring now to FIG. 4, therein is illustrated an equi alent capacitance circuit of the transformer of 1 16-3. The circuit is comprised of four serially connected capacitors 60, 61, 62 and 63 connected between the high voltage terminal 44 and ground. The capacitances oil-63. correspond to the capacitances through each of the wind ing groups Sal-34, respectively. A capacitance 64 is connected between the junction of capacitor 60 and 61 to ground, and the capacitance 64 represents the capacitance of the winding group 3 1 to ground. Similarly, capacitors 65 and 65 are connected to. the junction between ground and the junctions of capacitors 61 and 62, and the junc-v tion of capacitors 62 and 63, respectively, and the capacitors 65' and 66 are the capacitors that represent the capacitances of the windings groups 32 and 33 to ground respectively. The capacitor 60 is shunted by a capacitor 67 corresponding to the capacitance between the electrostatic shield 4'7 and pad 55 of winding group 32. Similarly a capacitance 68 shunting the capacitor 61 corresponds to the capacitance between the electrostatic shield 47 and pad 55 of winding group 33. Another capacitor 69 connected in shunt with the capacitor 62 corresponds to the capacitance between the electrostatic shield 47 and pad 55 of Winding group 34. The eifect of these capaci-v tances in respect to the distribution of voltage across the high voltage Winding will be disclosed in more detail in the following paragraphs.

Referring now to FIG. 5, therein is illustrated another modificatiton of the potential transformer of this invention. In this modification, the high voltage Winding groups, high voltage group of electrostatic shields and pads, and inter-connections between the windings and pads of the high voltage windings are identical with those in the embodiment of FIG. 3, and will not be further discussed with reference to this figure. Similarly the core' and low voltage winding (not illustrated for the sake of clarity of the drawing) may be identical with those of the arrangement of FIG. 3. In this arrangement the low voltage winding and the core are surrounded by an insulating cylinder 79 extending co-extensively with the high voltage winding, the cylinder 70 being closed at its upper end in a generally hemispherical manner. The highvoltage winding group 34 is wound directly on the lower end of the winding. cylinder 70. Another winding cylinder '71 is nested over the cylinder 70 and extends from the lower end of the winding group 33 upwardly,

and has a closed end generally hemispherical shape similar to that of the cylinder 7 t). The winding group 33 is wound directly on the lower end of the insulating cylinder 71, and the cylinder 71 may have a radially outwardly extending flange between the winding groups 33 and 3'4. A similar insulating cylinder 72 is nested over the cylinder 71 and the high voltage winding group 32 is wound directly on the lower end of the cylinder 72. The cylinder 72 may have a flange extending radially outwardly between the winding groups 32 and 33. Still another similarly shaped insulating cylinder 73 is provided extending from the winding group 3 1 upwardly, and the winding group 31 is found directly on the lower end of the cylinder 73. The cylinder 73 has a radially outwardly extending flange between the winding groups 31 and 32. An insulating tap 74 may be provided over the upper end of the insulating cylinder 73, the tap 74 having a'radially outwardly extending flange just above the winding group 31.

The radial outer surfaces of each of the insulating members 7li-74 are provided with conductive coatings in order to improve the electrostatic stressed distribution within the insulating structure. The coating on the cylinder 70 is connected to ground, the coating on the cylinder 71 is connected to the inner terminal of the winding group 33, the coating on the cylinder 72 is connected to the inner end of the winding group 32, and the coating on the cylinder 73 is connected to the inner end of the winding group 31. The high voltage terminal 44 is connected to the conductive coating on the tap insulating member 74.

While the insulating shell of the transformer illustrated in FIG. may be the same as that in FIG. 3, an alternate arrangement has been illustrated in FIG. 5 in which the shell and tap of the transformer are comprised of a singular insulating shell member 80 having a closed rounded upper end, with an aperture in the upper end through which the terminal 44 may extend. The use of such a shell may simplify the fabrication of the transformer.

. While the arrangements of FIG. 3 and FIG. 5 are electrically identical, the arrangement of FIG. 3 may be diflicult to form since some of the insulating cylinders have flanges at both ends, thereby necessitating the forming of some of the flanges after the windings are in place. In the arrangement of FIG. 5, however, all of the insulating cylinders may be completely formed before assembly of the transformer, since each insulating cylinder has only one radially extending flange. In each arrangement, however, it is to be noted that the insulation between the high and low voltage windings is progressively increased as the voltage increases from ground potential to line terminal. Thus, while full transmission system voltage insulation may be required between the high voltage winding group 31 and the low voltage winding, the insulation for the remaining high voltage winding groups may be progressively decreased.

Transformers of the type disclosed in the preceding paragraphs must be designed for surge voltages, which may result from lightning or switching as well as the normal transmission line voltage. In each group, such as 3 1, 32, 3-3 and 34 comprising a winding of the layer type having inner and outer shields, such as 46 and 47, the impulse or surge voltage distribution within the winding, that is between these shields, is linear and each layer is stressed in proportion to the applied voltage on each group.

When several windings of the layer type are connected in series however, the impulse voltage distribution among the groups is not uniform. Assuming that the transformer of FIGS. 3 and 5 is provided without the capacitance pads 55, when a surge due to lightning or switching is applied to the terminal 44, the voltage will not distribute uniformly among the various groups comprising the high voltage winding. The instantaneous distribution of the voltage among the groups is effected through the medium of their capacitances. The capacitance of the complete winding as illustrated in FIG. 4, consists of the series and parallel capacity elements 6066 comprising the capacities of each group from one terminal to the other and the capacities from each group to ground.

The charging of the various capacity elements to the respective voltages corresponding to the initial voltage distribution along the winding is effected by the flow of current in the above capacitance network.

If the initial voltage distribution thus produced among the groups does not have uniform voltage gradient, subsequent and more gradual changes take place in an effort to establish a uniform distribution. These changes are effected by the charging currents flowing from one capacity element to another throughout the inductance of the winding. It is well recognized that the voltage dis tribution under steady commercial frequency voltage is dictated by the inductance of the winding while under surges the voltages distribution among the groups is dictated by the block capacitance network of FIG. 4. As is further well known, such flow of current between capacity elements through inductance results in oscillations, the current surging back and forth with alternating voltage values above and below the values corresponding to a uniform voltage gradient. The amplitude of the oscillations will initially correspond to the difference between the initial voltage distribution and the final voltage distribution along the uniform gradient. These oscillations create successive voltage stresses between adjacent parts of the groups and between each group and ground. This dangerous initial voltage distribution and the oscillations resulting therefrom will not occur, however, if the initial voltage distribution among the groups is uniform. The initial voltage stresses and the oscillations resulting from the initial voltage distribution will be greatly reduced if the group to group capacities are increased. An increase in group to group capacitance is accomplished by the use of the previously described shielding pads 55 and their associated shields 47, which produce the so-called shunt capacitors 67, 68 and 69 illustrated in FIG. 4. The ca- .pacitors 6769 co-operate, respectively, with the capacitance of their associated high voltage winding groups, as represented by the capacitors 60-63 in FIG. 4, to increase the total capacitance across each respective winding group. The effect of this increase in capacitance is to lower the impedance of each winding group in proportion to its increase in capacitance, and thus proportionally lower the voltage drop across the'respective winding groups. Consequently, the relative voltage drops across the winding groups can be regulated by varying the capacitances 6769 to produce a uniform voltage distribution among the windings. Referring now to FIG. 4, when a voltage surge is suddenly applied to the high voltage terminal 44, the voltage drop from the terminal 44 to the ground side of the capacitor 64 will be equal to the voltage surge; the voltage drop across the capacitor 60 (i.e. the winding group 31) will be decreased because the parallel capacitor 67 (i.e. the effect produced by the associated shields 55 and 47) has reduced its effective impedance. Consequently, a larger proportion of the voltage surge will be applied to the capacitor 64 because of the effect of the shields, and this increased proportion of the voltage surge will be distributed among the other windings. The shields have a similar effect on the remaining windings. That is, the above referred to voltage drop across the capacitor 64 determines the voltage applied to the capacitors 61 and 65. Thus, the shields represented by the capacitor 68 have lowered the effective impedance across the winding group 31, as represented by the capacitor 61, so that a larger proportion of the voltage drop across the capacitors 61 and 65 is absorbed by the capacitor 65. In a similar manner, the voltage drop across the capacitor 65 determines the amount of the voltage surge applied to the capacitors 62 and 66, and the capacitor 69 causes a larger proportion of this surge to appear across the capacitor 66 and thus applied to the winding group 34, as represented in FIG. 4 by the capacitor 63. Thus, the effect of the shielding pads 55, as represented by the capacitors 6769, is to substantially uniformly distribute a voltage surge among the winding groups.

Referring now to FIG. '6, if a voltage surge is applied to the terminals of the high voltage winding of FIG. 3 and FIG. Sand in which the capacitances 6'769 are not provided, the voltage distribution among the groups 3 1, 32, 33 and 34 will be extremely non-uniform as illustrated by curve A in this figure. In this instance, it is noted that a disproportionately large portion of the initial voltage is impressed across the first group 31 of the high voltage winding of the transformer of FIGS. 3 and 5.

Approximately 90% of the initial voltage when a surge occurs thus appears across the first two winding groups of the high voltage winding, so that it would be necessary to provide sufficient insulation in these two winding groups alone to withstand most of the applied voltage. When the winding is equipped with electrostatic shields or pads 55, however, the initial voltage distribution will be much more uniform as illustrated in curve B of FIG. 6, and it is to be noted that this curve very closely approximates the optimum uniform distribution illustrated by a curve C of FIG. 6. Thus, when the shields or pads 55 are employed and connected as disclosed in the preceding paragraphs, the requirements for insulation of the high voltage winding groups to withstand surges is substantially reduced. When the surge voltage distribution among groups is uniform, or nearly so, the insulation to be provided between groups can be predetermined and supplied. The cascaded insulation disclosed in'the preceding paragraphs is adequate to withstand surges which may be impressed upon the transformer. The elfects produced by the cascading of insulation also co-operate with the effects of the above described shielding in' the manner described below.

As has been previously pointed out, the insulation between the high and low voltage winding groups is progressively increased as the voltage increases. Obviously this increases the space between adjacent high and low volt age winding groups. Since capacitance varies inversely as the spacing of the plates of the capacitor, increasing the insulation between the high and low voltage windings lowers the capacitance therebetween. Thus 'the capacitance between the high voltage winding groups and ground, as represented in FIGURE 4 by the capacitors 64, 65, and 66, is progressively decreased. Sinceimpedance varies inversely with capacitance, this means that the impedance from the high voltage windings to ground is progressively increased as voltage increases. Referring now to FIGURE 4 and in particular to the capacitor 64,, which represents the capacitance between the winding group 31 and ground, it will be seen that the increased insulation has caused a decrease in capacitance and hence an increase in impedance across the capacitor 64. Consequently, when a voltage surge is applied to the terminal 44, a relatively large proportion of the voltage surge will be absorbed across the capacitor 64 because of its in-. creased impedance. Thus the windings 31, as represented by the capacitor 60, will absorb less of the voltage surge. This same occurrence is repeated throughout the windings with respect to the capacitors 61 and 65, and the capacitors 62 and 66 as shown in FIGURE 4. That is, the increase in insulation between the winding 32 and ground causes the capacitor 65 to absorb a relatively larger proportion of the voltage surge across the capacitors 61 and 65, and likewise the capacitor 66 absorbs a larger part of the :surge across the capacitors 62 and 66 because of its increased impedance resulting from the increased. insulation. The overall result is thatthe increase in insulation augments-orcomplernents the shielding effect of the pads 55, as represented by the capacitors 67, 63 and 69. in FIGURE 4. Thus the shielding pads and the increased insulation work together to more uniformly distribute a voltage surge among the winding groups. Consequently, by integrating the effect produced by the above described expedients into a unified result, a transformer of uniquely simple construction with optimum obtainable characteristics for absorbing voltage surges can be economically produced.

It will be understood, of course, that while the forms of the invention herein shown and described constitute the preferred embodiments of the invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications thereof. It will also be understood that the words employed are words of description rather than limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed, and it is aimed in, the appended. claims to cover all such changes as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A transformer comprising a magnetic core, a low voltage winding surrounding said core, a high voltage winding surrounding said low voltage winding, said high voltage winding comprising a plurality of axially spacedapart serially connected winding groups, a high voltage.

terminal electrically connected to one of the axially endmost of said groups, said high voltage groups being progressively increasingly insulated from said low volt age winding toward said one group, electrostatic shield means for each high voltage winding group comprising anv inner shield and an outer shield, the inner shields being electrically connected in series with the low voltage end of the winding groups radially inwardly on said groups, and the outer shieldsbeing electrically connected in series with the high voltage end of the winding groups radially outwardly of said groups, and said electrostatic shield means further comprising a shielding pad foreach high voltage winding group except said one group, each shielding pad being electrically connected to the outer shield of the next higher voltage winding group, and each shielding pad being axially adjacent but spaced radially outwardly from its associated outer shield.

2. A transformer comprising a magnetic core, a low voltage winding surrounding said core, a high voltage winding surrounding said low voltage winding, said high voltage winding comprsing a plurality of axially spacedapart serially connected winding groups, a high voltage.

means for each high voltage winding group comprising an inner shield and an outer shield, the inner shields being electrically connected in series with the low voltage end of the winding groups radially inwardly on said groups, and the outer shields being electrically connected in series with the high voltage end of the winding groups radially outwardly of said groups, each pair of inner and outer shields being substantially axially coextensive with its associated high voltage winding group, and said, electrostatic shield means further comprising a shielding pad for each high voltage winding group except said one group, each shielding pad being electrically connected to the outer shield of the next higher voltage winding group, and each shielding pad being axially adjacent but spaced radially outwardly from its associated outer shield.

3. A transformer comprising a magnetic core, a low voltage winding surrounding said. core, ahigh voltage winding surrounding said low voltage winding, said high voltage winding comprsing a plurality of axially spaced apart serially connected winding groups, a high voltage terminal electrically connected to one of the axially endmost of said groups, the other axially endmost group being electrically connected to ground, said high voltage groups being progressively increasingly insulated from said low voltage winding toward said one group, electrostatic shield means for each high voltage winding group comprising an inner shield and an outer shield, the inner shields being electrically connected in series with the low voltage end of the winding groups radially inwardly on said groups, and the outer shields being electrically connected in series with the high voltage end of the winding groups radially outwardly of said groups, each pair of inner and outer shields being substantially axially coextensive with its associated high voltage winding group, and the inner shield of each high voltage winding group except said other endmost group being electrically connected to the outer shield of the next lower voltage group, the inner shield of said other endmost group being electrically connected to ground, said electrostatic shield means further comprising a shielding pad for each high voltage winding group except said one group, each shielding pad being electrically connected to the outer shield of the next higher voltage winding group, and each shielding pad being axially adjacent but spaced radially outwardly from its associated outer shield.

4. A potential transformer having a core comprising a stack of flat magnetic strips, alow voltage winding around said stack, a high voltage winding in adjacent said low voltage winding, said high voltage winding comprising a plurality of axially spaced apart serially connected winding groups, radially expanding means insulating said high voltage winding groups from said low voltage winding in such a manner that the insulation progressively increases as voltage increases, thus causing the capacitance between the high voltage winding groups and ground to progressively decrease as voltage increases, an axially endmost group of said high voltage windings being connected to a high voltage terminal, electrostatic shield means for each high voltage winding group comprising an inner shield and an outer shield, the inner shields being electrically connected in series with the low voltage end of the winding groups radially inwardly of said groups, and the outer shields being electrically connected in series with the high voltage end of the winding groups radially outwardly of said groups, said electrostatic shield means further comprising a shielding pad for each high voltage winding group except said axially endmost group, each shielding pad being electrically connected to the outer shield of the next higher voltage winding group, and each shielding pad being axially adjacent but spaced radially outwardly from its associated outer shield, whereby the effects of the progressively decreasing capacitance and electrostatic shielding of said high voltage windings combined to cause voltage surges applied to the high voltage end of said transformer to be absorbed by the windings in a substantially uniform optimum manner.

5. Apparatus as recited in claim 1 in which said insulating means comprises a first insulating element of predetermined length and thickness surrounding said low voltage winding, at least one additional insulating element of predetermined thickness surrounding said first insulating element, said additional insulating element, being shorter in length than said first insulating element, said additional insulating element having a flange portion extending radially outwardly therefrom, one of said high voltage winding groups surrounding said first insulating element and being separated from said low voltage winding by a distance at least as large as the thickness of said first insulating element, another of said high voltage Winding groups surrounding said additional insulating element and being separated from said low voltage winding by a distance at least as large as the sum of the thicknesses of said first and additional insulating elements, and said one and other high voltage winding groups being separated by the radially extending flange on said additional insulating element.

6. Apparatus as recited in claim 5 in which the insulating elements are open at both ends and have an axial bore extending therethrough, and said additional insulating element has radially outwardly extending flange portions at both ends thereof.

7. Apparatus as recited in claim 5 in which the insulating elements have an open end, a closed end, an axial bore extending therethrough, and said flange portion is located adjacent said open end.

8. Apparatus as recited in claim 7 in which the outer surfaces of said insulating elements are provided with a conductive coating, the conductive coating of each insulating element being connected to one of high voltage winding groups, and the conductive coating of said first insulating element being also connected to ground.

9. Apparatus as recited in claim 1 in which an outer insulating shell surrounds said transformer, said shell comprising a plurality of body portions joined together by external connections, said external connections being shielded by means circumscribing said insulating shell, the last mentioned means being electrically connected to said high voltage winding, whereby substantially uniform voltage distribution is produced at the exterior of said insulating shell.

10. A potential transformer having an open core of flat magnetic strips, a low voltage winding surrounding said core, the low voltage winding comprising a plurality of winding groups spaced axially of said core, a high voltage winding surrounding said low voltage winding, said high voltage winding comprising a plurality of serially connected winding groups spaced axially of said core at locations adjacent respective low voltage winding groups, a first high voltage group at one end of the winding being connected to ground and a last high voltage group at the other end of the winding being connected to a high voltage terminal, means progressively insulating and spacing respective high and low voltage winding groups from each other, said insulating and spacing means comprising a plurality of insulating elements of predetermined thickness, said insulating elements being open at each end and having an axial bore extending therethrough, the bores of respective insulating elements being progressively larger in diameter, the lengths of the respective insulating elements being progressively shorter as the bore diameters increase so that the insulating element with the largest bore is the shortest in length, the insulating elements being nested within one another so as to define a radial ledge portion on the exterior of each insulating element, said core and low voltage winding groups being located in the bore of the innermost of said insulating elements, the first high voltage winding group surrounding the ledge portion of said innermost insulating element and being separated from its respective low voltage winding by a distance not less than the thickness of said innermost insulating element, succeeding high voltage winding groups surrounding the ledge portions of the succeeding nested insulating elements so that the high voltage groups are spaced from their respective low voltage groups by progressively increasing distances, with the last high voltage group being spaced from its respective low voltage group a distance not less than the sum of the thicknesses of all of the insulating elements, thus causing the insulation between high and low voltage winding groups to progressively increase as the voltage within the transformer increases, while causing the capacitance between respective high voltage winding groups and ground to progressively decrease as the voltage increases, electrostatic shield means for each high voltage winding group comprising an inner shield and an outer shield, the inner shields being electrically connected in series with the low voltage end of the winding groups radially inwardly of said groups, and the outer shields being electrically connected in series with the high voltage end of the winding groups radially outwardly of said groups, and said electrostatic shield means further comprising a shielding pad for each high voltage winding group except said last group, each shielding pad being electrically connected to the outer shield of the next higher voltage Winding group, and each shielding pad being axially adjacent but spaced radially outwardly from its associated outer shield, whereby the eifects of the progressively decreasing capacitance and electrostatic shielding of said high voltage windings combine to cause voltage surges applied to said high voltage terminal to be absorbed by the windings in a substantially uniform optimum manner;

11. Apparatus as recited in claim in which each of said insulating elements has a first radially outwardly projecting flange portion at the end thereof that is nearest to said high voltage terminal, and each of said insulating elements, except said innermost insulating element, has a second radially outwardly projecting flange portion at its opposite end, said first flange portions nesting within one another so as to provide multiple layers of insulation between said last high voltage group and said high voltage terminal, and said second flange portions axially separating said high voltage winding groups.

12. A potential transformer having an open core of flat magnetic strips, a low voltage winding surrounding said core, the low voltage winding comprising a plurality of winding groups spaced axially of said core, a high voltage winding surrounding said low voltage winding, said high voltage winding comprising a plurality of serially cgnnected'winding groups spaced axially of said core at locations adjacent respective low voltage Winding groups, a first high voltage group at one end of the winding being connected to ground and a-last high voltage group at the other end of the winding being connected to a high voltage terminal, means progressively insulating and spacing respective high and low voltage winding groups from each other, said insulating and spacing means comprising a plurality of insulating elements of pro-determined thickness, said insulating elements being open at one end, closed at the other end having an axial bore extending therethrough, the bores of respective insulating elements being progressively larger in diameter, the lengths of the respective insulating elements being progressively shorter as the bore diameters increase so that the insulating element with the largest bore is the shortest in length, the insulating elements being nested within one another so as to define a radial ledge portion on the exterior of each insulating element, said core and low voltage winding groups being located in the bore of the innermost of said insulating elements, the first high voltage winding group surrounding the ledge portion of said innermost insulating element and being separatedfrom its respective low voltage winding by a distance not less than the thickness of said innermost insulating element, succeeding high voltage winding groups surrounding the ledge portions of the succeeding nested insulating elements so that the high voltage groups are spaced fromtheir respective low voltage groups by progressively increasing distances, With the last high voltage group being spaced from its respective low voltage group a distance not less-than the sum of the thicknesses of all of the insulating elements, thus causing the insulation between high and low voltage winding groups to progressively increase as the voltage within the transformer increases, while causing the capacitance between respective high voltage winding groups and ground to progressively decrease as the voltage increases, electrostatic shield means for each high voltage winding group comprising an inner shield and an outer shield, the inner shields being electrically connected in series with the low "volt g e d 0 the winding groups radially inwardly ofsaid groups, andthe outer shields being electrically connected in series with the high voltage end of the winding groups radially outwardly of said groups, and said electrostatic shield means further comprising a shielding pad for each high voltage winding group except said last group, each shielding pad being electrically connected to the outer shield of the next higher voltage winding group, each shield beingaxially adjacent but spaced radially outwardly from its associated outer shield, whereby the effects of the progressively decreasing capacitance and electrostatic shielding of said high voltage windings combine to cause voltage surges applied to said high voltage terminal to be absorbed by the windings in a substantially uniform optimum manner.

13. Apparatus as recited in claim 12 in which each of said insulating elements, except said innermost insulating element, has a radially outwardly projecting flange portion at its open end, said flange portions axially separating said high voltage winding groups.

14. A transformer having a core, a low voltage winding surrounding said core, a high voltage winding comprising a plurality of axially spaced apart winding groups adjacent said low voltage winding, each winding group comprising a plurality of concentric layers of an electrical conductor with each layer having a plurality of axially aligned turns, each winding group having a low voltage end anda high voltage end, said winding groups being connected in series, a first high voltage winding group being connected at its low voltage end to ground, and a last high voltage group being connected at its high voltage said last group, each shielding pad being electrically connected to the outer shield of the next higher voltage winding group, and each shielding pad being axially adjacent but spaced radially outwardly from its associated outer shield.

15. Apparatus as recited in claim 14 in which said insulating means comprises a plurality of insulating elements having a conducting coating on the external surface thereof, and each of said high voltage winding groups is connected at its low voltage end to the coating on one of said insulating elements.

16. Apparatus as recited in claim 14 in which said transformer is a potential transformer having an open magnetic core comprising a single stack of flat magnetic strips.

References Cited in the file of this patent UNITED STATES PATENTS 1,899,720 Putnam Feb.' 28, 1933 1,916,588 Schrader July 4, 1933 1,940,864 Hodnette Dec. 26, 1933 1,973,073 Hodnette Sept. 11, 1934 2,279,027 Weed Apr. 7, 1942 FOREIGN PATENTS 294,051 Switzerland Jan. 4, 1954 625,271 Germany Feb. 7, 1 936 

